Light olefins (defined as "ethylene, propylene, and butylene") serve as feeds for the production of numerous chemicals. Light olefins traditionally are produced by petroleum cracking. Because of the limited supply and/or the high cost of petroleum sources, the cost of producing olefins from petroleum sources has increased steadily.
Alternative feedstocks for the production of light olefins are oxygenates, such as alcohols, particularly methanol, dimethyl ether, and ethanol. Alcohols may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for olefin production.
The catalysts used to promote the conversion of oxygenates to olefins are molecular sieve catalysts. Because ethylene and propylene are the most sought after products of such a reaction, research has focused on what catalysts are most selective to ethylene and/or propylene, and on methods for increasing the selectivity of molecular sieve catalysts to ethylene and/or propylene. The selectivity of certain molecular sieve catalysts to ethylene and propylene is known to increase if the level of coke on the total reaction volume of molecular sieve catalyst is maintained in the range of from about 2 wt % to about 30 wt %. Some have suggested maintaining this desired level of coke by removing all or a portion of the total reaction volume of catalyst, partially regenerating the catalyst so removed, and returning the partially regenerated catalyst to the reactor. However, partial regeneration may not result in maximum selectivity of the catalyst to light olefins.
Methods are needed which will maintain a desired level of coking on molecular sieve catalysts during the conversion of oxygenates to olefins while maintaining maximum activity of the catalyst.